How to calculate average speed.

OK this might be a stupid question (most probably is) but I thought I should confirm it just the same. Lets say that on my quest to learn how to correctly expose slide film I want to try the average rule. Using the camera spot meter I get a reading for highlights/midtones/shadows and I want to use the average.

So, I get 1/500 for highlights, 1/125 for midtones and 1/8 for the shadows (figures theoretical) what would be the average speed to use, 1/211 (meaning the actual closest possible) correct?

If it's 35mm, use a Nikon F4/F5 or equivalent as their matrix metering was made for exposing slide film. Otherwise just bracket and/or use an incident meter. The OPs light measuerments are right on the edge of what I'd expect out of slide film; any more range and I'd use negative film like Portra for it's bigger capture range. It wouldn't be a slide, but it's recorded on film.

The incident meter will give an average reading of light falling on the subject.

With slides, you main concern is not overexposing the highlights. The shadows will just have to fall where they will. There's not enough latitude in slide film nor can your vary development times to extend or reduce density Zone system style.

That example averages the highlight and the shadow points from a spot meter. If you then also want to consider the mid-tone point you will need to decide if it is worth giving up some of the shadow detail to place the mid tones closer to where you measured them.

The highlight and shadow point average in your example is asking for more exposure than your mid tone measurement. Which setting is better is simply a matter of taste. To test and see which you like better from this example I would shoot two frames, one at 1/60, the other at 1/125 then compare.

Mark Barendt, Beaverton, OR

"We do not see things the way they are. We see things the way we are." Anaïs Nin

The incident meter will give an average reading of light falling on the subject.

With slides, you main concern is not overexposing the highlights. The shadows will just have to fall where they will. There's not enough latitude in slide film nor can your vary development times to extend or reduce density Zone system style.

For me there is a bit more nuance here, the exposure I want for the faces in the scene always trumps the rest of the composition.

Mark Barendt, Beaverton, OR

"We do not see things the way they are. We see things the way we are." Anaïs Nin

For slide film, you need to base your exposure on the highlights, not average. Measure the brightest white that you still want a little detail in in your scene, then open up two stops. That should get you in the ball park. Make notes about your meter and adjust a bit if this isn't giving you exactly what you need in the way of highlight density. Shadows fall where they may.

Another way of putting it is that the "average" is not the relevant measurement. You want to make sure that the "important" parts of the image are in the useful range of the film, which for slide film is narrower than for negative film. Depending on your image, the "important" bit might be a face. Interesting application: when shooting steam locomotives outdoors, I found that the metering the locomotive directly with a standard center-weighted meter in an SLR would render the black locomotive at 18% grey. A better metering, using only the camera meter, was to meter on the "ballast" rocks or green grass in the foreground, which would render the locomotive an appropriate shade of black with detail.